Method and apparatus for the continuous production of diaryl carbonates

ABSTRACT

An energy efficient series of mass and energy integrated reactive distillation columns and distillation columns are used to effect the production of diaryl carbonate. Utilizing the method or apparatus of the invention facilitates high diaryl carbonate production rates, and convenient recovery of unreacted starting materials and side-reaction products for recycle within the process for making diaryl carbonates or utilization in parallel reactions such as the manufacture of dialkyl carbonates. The method makes use of three reactive distillation columns and two rectification columns which are joined by a plurality of lines for transferring reactants and/or products into and out of the columns.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

FEDERALLY SPONSORED RESEARCH

Not applicable

BACKGROUND OF THE INVENTION

This application relates to the continuous production of diarylcarbonates by reaction of dialkyl carbonates and an aromatic alcohol inthe presence of a catalyst.

Diaryl carbonates, such as diphenyl carbonate, are an important reactantin the production of polycarbonate resins. As the uses to whichpolycarbonates are put have increased, the efficient production ofdiaryl carbonates has become of greater significance. Early processesfor the production of diaryl carbonates utilized phosgene as a reagent.The toxicity of phosgene, however, prompted the development of anon-phosgene process. As shown schematically in FIG. 1, the non-phosgeneprocess involves a two-step process. First, a dialkyl carbonate such asdimethyl carbonate (DMC) reacts with an aromatic alcohol such as phenolto produce an alkyl aryl carbonate (e.g., phenylmethyl carbonate) and analkyl alcohol (methanol). Then, in the second step, two molecules of thealkyl aryl carbonate undergo a transesterification reaction to produceone molecule of diaryl carbonate (diphenyl carbonate, DPC) and onemolecule of dialkyl carbonate (DMC).

Various methods and apparatus for making diaryl carbonates are known inthe art. For example, U.S. Pat. No. 5,210,268, which is incorporatedherein by reference, relates to a process for continuously producingaromatic carbonates. The process is carried out in a distillationcolumn, with product being recovered from the bottom of the column, andlow boiling by-products being removed via the top of the column. Otherprocesses for production of diaryl carbonates using a series ofdistillation columns are disclosed in U.S. Pat. Nos. 5,344,954 and5,705,673 which are incorporated herein by reference.

The reaction shown in FIG. 1 is the reaction which is desired, but as isknown to persons skilled in the art, there are number of side reactionswhich occur, producing unwanted by-products. These by-products caninterfere with continuing production of the desired product, reduce theefficiency of the over-all process, and in some cases produce wastestreams which require special handling for disposal. Thus, a significantchallenge to the utilization of this process is the development of aprocess which minimizes the quantities and effects of the reactionby-products, while providing a good yield of the desired product.

The present invention provides a method for continuous production ofdiphenyl carbonate which has a high production rate while at the sametime maintaining an energy efficient process. The present inventionfurther provides an apparatus for continuous production of diphenylcarbonate which has a high production rate while at the same timemaintaining an energy efficient process.

SUMMARY OF THE INVENTION

The invention is directed to a method and apparatus for continuousproduction of diaryl carbonates by reaction of a dialkyl carbonate andan aromatic alcohol in the presence of a transesterification catalyst.The method comprises the steps of:

(a) introducing reactant streams containing dialkyl carbonate, aromaticalcohol and transesterification catalyst to a first reactivedistillation column to produce alkyl aryl carbonate and alkyl alcohol;

(b) recovering from the first reactive distillation column a first topstream containing dialkyl carbonate and alkyl alcohol and a first bottomstream containing alkyl aryl carbonate;

(c) introducing the first bottom stream into a second reactivedistillation column to produce diaryl carbonate by disproportionation ofthe alkyl aryl carbonate;

(d) recovering from the second reactive distillation column a first sidestream containing dialkyl carbonate and alkyl aryl ether and a secondbottom stream containing diaryl carbonate, alkyl aryl carbonate anddialkyl carbonate;

(e) introducing the first side stream into a second rectification columnto separate a dialkyl carbonate stream from the alkyl aryl ether, andrecycling the dialkyl carbonate stream to the first rectificationcolumn;

(f) introducing the second bottom stream to a third reactivedistillation column to further drive the reaction toward diarylcarbonate;

(g) recovering from the third reactive distillation column a second topstream containing unreacted aromatic alcohol, dialkyl carbonate andalkyl aryl ether and recycling the second top stream to the firstreactive distillation column;

(h) introducing the first top stream into a first rectification column;

(i) recovering from the first rectification column an azeotrope topstream consisting essentially of dialkyl carbonate/alkyl alcoholazeotrope and a third bottom stream containing dialkyl carbonate, andrecycling the third bottom stream to the first reactive distillationcolumn; and

(j) recovering a product stream containing essentially all of the diarylcarbonate produced from the bottom of the third reactive distillationcolumn.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the two-step reaction of dimethyl carbonate and phenol toproduce diphenyl carbonate; and

FIG. 2 shows an apparatus in accordance with the inventionschematically.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an energy efficient series of mass andenergy integrated reactive distillation columns and distillation columnsto effect the production of diaryl carbonate. Utilizing the method orapparatus of the invention facilitates high diaryl carbonate productionrates, and convenient recovery of unreacted starting materials andside-reaction products for recycle within the process for making diarylcarbonates or utilization in parallel reactions such as the manufactureof dialkyl carbonates.

The principal reaction which is carried out in the method of the presentinvention is the reaction illustrated in FIG. 1. It will be appreciatedby persons skilled in the art that this process can be carried out usingvarious dialkyl carbonates and various aromatic alcohols. Exemplarymaterials of each type are listed in the patents discussed above. Sincethe most common reactants used industrially are dimethyl carbonate andphenol, which react to produce diphenyl carbonate, however, thesematerials will be used as examples throughout the following discussionof the invention which follows. It should be understood, however, thatthis usage is merely for clarity of discussion, and that no limitationon the invention to the use of these specific materials is intended.

FIG. 2 shows a schematic representation of the apparatus of theinvention, including five columns A, B1, B2, B3 and C. Various feed,product and recycle streams indicated by the numbers 1-18.

Columns A, B1 and C are reactive/distillation columns. Thus, thesecolumns each have a lower reaction section in which a chemical reactionoccurs, and an upper rectification section. The construction of columnsof this type are known in the art. In general, the reactive portion ofthe column will be furnished with arranged packings, dumped packings orfixed internals to provide at least three theoretical distillationstages. Preferably, the reactive section of Column A will provide 10 to60, and more preferably 15 to 40, theoretical distillation steps.

Columns B2 and B3 are rectification columns. Thus, these columns areintended to carry out a separation of materials based upon boilingpoint, without driving a concurrent chemical reaction. The constructionof columns of this type are known in the art.

The five columns illustrated in FIG. 2 are interconnected by a series offeed/recycle lines which serve to transport reactants and products. Thedirection of flow for each such line is indicated on FIG. 2. Variousvalves, heaters, and other fittings may be included with thesefeed/recycle lines in adapting the design to a particular installation,and the inclusion of such components is within the skill in the art.

The apparatus depicted in FIG. 2 can be utilized in accordance with themethod of the invention to produce DPC. Starting materials areintroduced to column A through as streams 3 and 4. Stream 3 is acombination of stream 1, which contains mainly phenol, either fresh orrecycled, and stream 3′ which contains phenol and catalyst. Optionally,stream 3 can also include DMC and side-reaction products recycled fromreactive distillation column C via line 16. Stream 4 is a mixture ofstream 2, containing mainly DMC and recycle stream 8 which containsmainly DMC and some minor amounts of phenol and side-reaction productsrecovered from the bottom of rectification column B2.

Stream 4 is fed into the bottom section of column A, preferably to thereboiler. The stream may be a liquid or a vapor, depending on the typeof reboiler used. For example, if an external reboiler, e.g., a kettlereboiler, is used, stream 4 enters column A as a vapor. Stream 3 is fedas a liquid into the middle section of column A, at a location at thetop of the reactive distillation section. The feed rate of streams 3 and4 is such that the molar ratio of aromatic alcohol to dialkyl carbonatewhich is introduced into column A is between 0.1 and 10, preferablybetween 0.5 and 5 and most preferably between 1 and 3. It isparticularly advantageous to provide dialkyl carbonate in excess tlroughstream 4, since dialkyl carbonate serves as both a reactant and astripping agent which facilitates removal of alkyl alcohol produced inthe transesterification reaction. This removal increases the rate ofproduction of alkyl aryl carbonate in column A.

The transesterification reaction in column A is carried out at atemperature from 100° C. to 300° C., preferably from 130° C. to 250° C.,and most preferably from 140° C. to 220° C. The operating pressure atthe top of column A is suitably in the range of 50 mbar to 20 bar,preferably 0.5 to 10 bar, and most preferably 3 to 7 bar.

Reaction products and unreacted starting materials are removed fromcolumn A in continuous maimer through streams 5 and 6. Stream 5, whichis drawn from the head of column A contains unreacted dialkyl carbonateand virtually all of the alkyl alcohol produced in thetransesterification reaction. This stream is passed to rectificationcolumn B2 for processing and recovery. Stream 6 which is drawn from nearthe bottom of column A contains the alkyl aryl carbonate produced incolumn A, in combination with unreacted starting materials and catalyst.This stream 6 is passed to the second reactive distillation column B1.

Column B1 has a lower reaction section and an upper rectificationsection. This column promotes the disproportionation of alkyl arylcarbonate into diaryl carbonate and dialkyl carbonate, while at the sametime separating dialkyl carbonate from the reaction mixture.

The reactive and rectification sections of column B1 are each furnishedwith arranged packings, dumped packings or fixed internals to provide 1to 50, preferably 5 to 20 theoretical distillation steps. Thetemperature profile of column B1 ranges from 50 to 300° C., preferably60 to 280° C., and most preferably 100 to 250° C.

The pressure in column B1 is maintained in the range of 50 mbar to 10bar, preferably 0.2 to 5 bar, and most preferably 1 to 3 bar. It isdesirable to maintain the pressure of column B1 below the pressure ofcolumn A. This results in an adiabatic flash of stream 6, hencefacilitating disengagement of dialkyl carbonate from the reactionmixture.

Column B1 is operated in such a way that dialkyl carbonate entering thecolumn through stream 6 is separated from the reaction mixture, henceincreasing the rate of the disproportionation reaction taking place inthe reactive section. Column B1 can also be utilized as a reboiler forcolumn B2, in which case the two columns are connected by streams 9 and11 as shown in FIG. 2. In this case, care should be taken to avoidcarryover of alkyl aryl carbonate to column B2 in this configuration,since this could result in recycle of alkyl aryl carbonate to column Avia stream 8. This would drive the composition in column A towards thestarting materials, hence lowering the net production rate of alkyl arylcarbonate in column A. Thus, columns B1 and B2 are operated such thatstream 9, when present, contains mainly dialkyl carbonate in the liquidphase, refluxing back from rectification column B2. Stream 11, whenpresent, contains mainly dialkyl carbonate and the unwanted byproductalkyl aryl ether (for example anisole) in the vapor phase. This providesmost of the energy to drive the separation processes taking place in thecolumn B2. Therefore, heat and mass integration is realizedadvantageously between columns B1 and B2 via streams 9 and 11.

Rectification column B2 produces a by-product stream 7 containing anazeotropic mixture of dialkyl carbonate and essentially all of the alkylalcohol produced in the process. This stream can be condensed and reusedas a feed stream for a complementary dialkyl carbonate productionprocess without further purification.

Column B2 is furnished with arranged packings, dumped packings or fixedinternals to provide at least 3 and preferably 5 to 50 theoreticaldistillation steps. The temperature profile of column B2 ranges from 10to 200° C., preferably 50 to 150° C. The operating pressure in column B2is in the range of 0.1 to 10 bar, preferably 0.5 to 2 bar.

In addition to streams 9 and 11 which interchange materials with columnB2, materials leave column B1 via streams 10 and 12. Stream 12 containsmainly dialkyl carbonate and alkyl aryl ether and is drawn off as a sidestream from column B1 and fed to a second rectification column B3.Column B3 separates dialkyl carbonate from alkyl aryl ethers, andreturns the dialkyl carbonate to column B2 via line 14. The alkyl arylethers are discharged through line 13. This separation of alkyl arylethers such as anisole is important, since these products can build upwithin the apparatus if not removed.

Stream 10 contains the diaryl carbonate produced in column B1, incombination with unreacted starting materials, and some alkyl arylcarbonate and alkyl aryl ethers. Stream 10 is fed to reactivedistillation column C, which is operated to further drive the reactiontoward the desired diaryl carbonate product, while separating othermaterials for recycle. Two streams are removed from column C. The firstis a product stream 15 which contains essentially all of the diarylcarbonate produced together with residual catalyst, some alkyl arylcarbonate and unwanted high boiling by-products.

This product stream 15 may be further distilled if additionalpurification is desired. The second stream 16 is removed from the headof column C as a recycle stream containing essentially all of theunreacted aromatic alcohol starting material, and some dialkyl carbonateand alkyl aryl ether, and recycled to make up part of stream 3.

Column C is suitably operated at a temperature of from 100 to 300° C.,preferably 100 to 250° C., and most preferably 140 to 200° C. Theoperating pressure in the column is suitably 10 mbar to 3 bar,preferably from 50 mbar to 1 bar, and most preferably from 100 to 400mbar.

Within the scope of the process noted above, several variations arepossible. Thus, as already described, the interconnection of columns B1and B2 via streams 9 and 11 is optional although it provides improvedefficiency. Similarly, stream 10 may be augmented by addition of astream containing alkyl aryl carbonate via stream 17 to form stream 18.This results in an improvement in the overall production of diarylcarbonate. Suitably, stream 17 may be a alkyl aryl carbonate-containingstream recovered from the purification of diaryl carbonates.

The method as described above is suitably practiced in an apparatus inaccordance with the invention. This apparatus comprises first, secondand third reactive distillation columns, and first and secondrectification column and a plurality of lines for transporting reactantand product streams, wherein:

(a) the first reactive distillation column is connected to input linesfor the introduction of reactants, and to first and second transferlines, said first transfer line running from the top of the firstreactive distillation column to the middle of the first rectificationcolumn and the second transfer line running from the bottom of the firstreactive distillation column to the second reactive distillation column(e.g., the bottom of the second reactive distillation column);

(b) the second reactive distillation column is connected to third andfourth transfer lines, said third transfer line running from the side ofthe second reactive distillation column to the top of the secondrectification column, and said fourth transfer line running from thebottom of the second reactive distillation column to the third reactivedistillation column (e.g. the bottom of the third reactive distillationcolumn);

(c) the third reactive distillation column is connected to a firstoutput line for providing diaryl carbonate product from the bottom ofthe third reactive distillation column and a first recycle line runningfrom the top of the third reactive distillation column to the middle ofthe first reactive distillation column;

(d) the first rectification column is connected to a second product linefor providing dialkyl carbonate/alkyl alcohol azeotrope from the top ofthe first rectification column, and a second recycle line running fromthe bottom of the first rectification column to the bottom of the firstreactive distillation column; and

(e) the second rectification column is connected to a third product linefor recovering alkyl aryl ethers from the bottom of the secondrectification column and a third recycle line running from the top ofthe second rectification column to the bottom of the first rectificationcolumn.

The apparatus may further comprise additional transfer lines running inopposing directions between the bottom of the first rectification columnand the top of the second reactive distillation column and/or anaugmentation line connected to the fourth transfer line for introductionof an augmenting reactant stream into the third reactive distillationcolumn.

It will be appreciated by persons skilled in the art that thepositioning of the various lines as described above as being in the top,middle or bottom of the column is necessarily a relative term since theposition at which material is to be introduced is dependent on theconditions being maintained in the column. For example, a line enteringthe bottom of the column may actually enter a few stages above the sump,and a line entering the top of the column may enter a few stages belowthe top stage. Nonetheless, these terms are included to define thegeneral orientation of the various columns and lines.

The method and apparatus of the invention allow the continuousproduction of diaryl carbonates via a catalyzed transesterification toproceed in a highly efficient manner on an industrial scale. Themultistage process and apparatus of the invention provide productionrates that are higher than those known in the art, and efficientseparation and recycle (where appropriate) of unreacted startingmaterials and reaction by-products. Thus, the present inventionrepresents an improvement over prior processes.

What is claimed is:
 1. A method for continuous production of diarylcarbonates by reaction of a dialkyl carbonate and an aromatic alcohol inthe presence of a transesterification catalyst comprising the steps of:(a) introducing reactant streams containing dialkyl carbonate, aromaticalcohol and transesterification catalyst to a first reactivedistillation column to produce alkyl aryl carbonate and alkyl alcohol;(b) recovering from the first reactive distillation column a first topstream containing dialkyl carbonate and alkyl alcohol and a first bottomstream containing alkyl aryl carbonate; (c) introducing the first bottomstream into a second reactive distillation column to produce diarylcarbonate by disproportionation of the alkyl aryl carbonate; (d)recovering from the second reactive distillation column a first sidestream containing dialkyl carbonate and alkyl aryl ether and a secondbottom stream containing diaryl carbonate, alkyl aryl carbonate anddialkyl carbonate; (e) introducing the first side stream into a secondrectification column to separate a dialkyl carbonate stream from thealkyl aryl ether, and recycling the dialkyl carbonate stream to a firstrectification column; (f) introducing the second bottom stream to athird reactive distillation column to further drive the reaction towarddiaryl carbonate; (g) recovering from the third reactive distillationcolumn a second top stream containing unreacted aromatic alcohol,dialkyl carbonate and alkyl aryl ether and recycling the second topstream to the first reactive distillation column; (h) introducing thefirst top stream into a first rectification column; (i) recovering fromthe first rectification column an azeotrope top stream consistingessentially of dialkyl carbonate/alkyl alcohol azeotrope and a thirdbottom stream containing dialkyl carbonate, and recycling the thirdbottom stream to the first reactive distillation column; and (j)recovering a product stream containing essentially all of the diarylcarbonate produced from the bottom of the third reactive distillationcolumn.
 2. The method of claim 1, further comprising the steps ofrecovering a liquid bottom stream from the first rectification columnand introducing the liquid bottom stream to the top of the secondreactive distillation column; and recovering a vapor stream containingdialkyl carbonate and introducing the vapor stream to the bottom of thefirst rectification column, whereby the second reactive distillationcolumn serves as a reboiler for the first rectification column.
 3. Themethod of claim 1, further comprising the step of augmenting the secondbottom stream with an additional stream containing alkyl aryl carbonate.4. The method of claim 1, wherein the first reactive distillation columnis maintained at a temperature of from 100° C. to 300° C., and apressure at the top of the column in the range of 50 mbar to 20 bar. 5.The method of claim 4, wherein the second reactive distillation columnis maintained at a temperature of from 50 to 300° C., and a pressure offrom 50 mbar to 10 bar.
 6. The method of claim 5, wherein the secondreactive distillation column is maintained at a pressure which is lowerthan the pressure of first reactive distillation column.
 7. The methodof claim 1, wherein the second reactive distillation column ismaintained at a temperature of from 50 to 300° C., and a pressure offrom 50 mbar to 10 bar.
 8. The method of claim 7, wherein the secondreactive distillation column is maintained at a pressure which is lowerthan the pressure of first reactive distillation column.
 9. The methodof claim 1, wherein the third reactive distillation column is maintainedat a temperature of from 100 to 300° C., and a pressure of from 10 mbarto 3 bar.
 10. The method of claim 9, wherein the first reactivedistillation column is maintained at a temperature of from 100° C. to300° C., and a pressure at the top of the column in the range of 50 mbarto 20 bar.
 11. The method of claim 10, wherein the second reactivedistillation column is maintained at a temperature of from 50 to 300°C., and a pressure of from 50 mbar to 10 bar.
 12. The method of claim11, wherein the second reactive distillation column is maintained at apressure which is lower than the pressure of first reactive distillationcolumn.